The invention relates to optical pulse transmission line parts which are particularly, but not exclusively, for use in high capacity, wavelength division multiplexed (WDM) optical waveguide transmission lines employing return-to-zero (RZ) optical pulses. The invention also relates to components for parts of an optical pulse transmission line, and to an optical pulse transmission line.
At high bit rates optical communication systems suffer degradation of their transmission capacity. This is mainly due to the detrimental effects on the optical pulses of chromatic dispersion, fibre non-linearity and noise from optical amplifiers in the system.
In the transmission of soliton pulses, positive use is made of the fibre non-linearity by achieving a balance with the pulse broadening due to dispersion in the region of the anomalous chromatic dispersion. Conventional soliton transmission lines comprise optical fibre which has a constant, or slightly varying, dispersion coefficient. However, the distance over which soliton pulses may be transmitted along such a transmission line is limited by two contradicting requirements: the dispersion must be low in order to minimise Gordon-Haus timing jitter, which is driven by optical amplifier noise; and the dispersion must be high in order to suppress four-wave mixing in wavelength division multiplexed transmission systems. This contradiction is resolved by using a technique known as dispersion management in which the optical fibre transmission line has high local dispersion and low path-average dispersion.
One of the major factors limiting optical data transmission distance and capacity in WDM systems is the effect of dispersion slope, which is a wavelength dependent parameter. The dispersion slope of an optical waveguide causes a variation in the dispersion parameters of the waveguide as a result of a change in the wavelength of an optical signal propagating along the waveguide. That is to say, the dispersion experienced by an optical signal propagating along the waveguide is dependent upon the wavelength of the optical signal. Therefore each optical channel in a WDM system will experience a different dispersion parameter. As a result it is difficult to achieve system optimisation for all wavelength channels simultaneously.
Existing approaches to compensating for the effect of dispersion slope are rather complex. These include per channel dispersion slope compensation, the manufacture of pairs of optical fibres having opposite values of dispersion and dispersion slope, and the manufacture of compensating optical fibres capable of compensating for both the dispersion and dispersion slope of known transmission fibres, such as standard monomode fibre (SMF).
According to a first aspect of the present invention, there is provided an optical pulse transmission line portion having dispersion slope compensation, the transmission line portion comprising:
a first optical waveguide of a first length (L1) having a first dispersion parameter (D1) and a dispersion slope parameter (S1) of a first sign,
optically coupled to
first dispersion slope compensating means of a second length (L2) having a second dispersion parameter (D2) and a dispersion slope parameter (S2) of the opposite sign,
wherein the path average dispersion slope of the transmission line portion is substantially zero and is given by
 less than S greater than =S1L1+S2L2
and the path average dispersion ( less than D greater than ) of the transmission line portion is generally non zero, and is given by       ⟨    D    ⟩    =                              D          1                ⁢                  S          2                    -                        D          2                ⁢                  S          1                                    S        2            -              S        1            
The path average dispersion of the transmission line portion is preferably wavelength independent.
The optical pulse transmission line portion is preferably for inclusion in a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero optical signal.
The first optical waveguide and the first dispersion slope compensating means are desirably arranged in a precompensating dispersion map configuration, or may alternatively be arranged in a postcompensating dispersion map configuration.
The transmission line portion preferably further comprises an optical amplifier. The optical amplifier may be optically coupled to the dispersion map at one end of the dispersion map or at a location along the dispersion map.
The transmission line portion may further comprise a second optical waveguide of a third length (L3) having a first dispersion parameter (D1) and a dispersion slope parameter (S1) of a first sign and a second dispersion slope compensating means of a fourth length (L4) having a second dispersion parameter (D2) and a dispersion slope parameter (S2) of the opposite sign.
The first and second optical waveguides and the first and second dispersion slope compensating means are desirably optically coupled together in a symmetric dispersion map configuration. The transmission line portion may further comprise a second optical amplifier. Desirably, one of the first and second optical amplifiers is provided at an end of the dispersion map and the other of the first and second optical amplifiers is provided towards the middle of the dispersion map.
The or each optical amplifier may be a fibre amplifier, such as an erbium doped fibre amplifier, or a fibre Raman amplifier. The or each optical amplifier may alternatively be a semiconductor optical amplifier device.
The first and second optical waveguides are preferably sections of a first optical fibre. The first optical fibre may be a transmission fibre such as standard monomode fibre, Lucent 2-wave fibre or Alcatel Terralight fibre.
The first and second dispersion slope compensating means preferably comprise sections of a third optical waveguide, which is most preferably a second optical fibre. The second optical fibre may be a compensating fibre, such as dispersion compensating fibre or reverse dispersion fibre.
The first and second dispersion slope compensating means may alternatively or additionally comprise sections of a fourth optical waveguide including an optical grating. The fourth optical waveguide is preferably a third optical fibre. The optical grating is preferably an optical fibre grating, such as a fibre Bragg grating.
According to a second aspect of the present invention, there is provided optical pulse transmission line portion components for incorporation into an existing terrestrial communication line which comprises a first optical waveguide of a first length (L1) having a first dispersion parameter (D1) and a dispersion slope parameter (S1) of a first sign, the components comprising:
first dispersion slope compensating means of a second length (L2) having a second dispersion parameter (D2) and a dispersion slope parameter (S2) of the opposite sign,
the first dispersion slope compensating means being connectable in optical communication with the first optical waveguide to form therewith an optical pulse transmission line portion having compensated dispersion slope,
wherein the path average dispersion slope of the transmission line portion is substantially zero and is given by
 less than S greater than =S1L1+S2L2
and the path average dispersion ( less than D greater than ) of the transmission line portion is generally non zero, and is given by       ⟨    D    ⟩    =                              D          1                ⁢                  S          2                    -                        D          2                ⁢                  S          1                                    S        2            -              S        1            
The path average dispersion of the transmission line portion is preferably wavelength independent.
The optical pulse transmission line portion preferably forms part of a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero signal.
Desirably the first optical waveguide and the first dispersion slope compensating means, when connected together, form a precompensating dispersion map, or may alternatively form a postcompensating dispersion map.
The transmission line portion components preferably further comprise a first optical amplifier, being connectable in optical communication with the optical pulse transmission line portion. The first optical amplifier may be connected in optical communication with the transmission line portion at one end of the dispersion map or at a location along the dispersion map.
The existing terrestrial communication line may further comprise a second optical waveguide of a third length (L3) having a first dispersion parameter (D1) and a dispersion slope parameter (S1) of a first sign.
The first and second optical waveguides are preferably sections of a first optical fibre. The first optical fibre may be a transmission fibre such as standard monomode fibre, Lucent 2-wave fibre or Alcatel Terralight fibre.
The transmission line portion components may further comprise a second dispersion slope compensating means of a fourth length (4) having a second dispersion parameter (D2) and a dispersion slope parameter (S2) of the opposite sign, and being connectable in optical communication with one or more of the aforementioned components, and the first and second optical waveguides.
Desirably, the first and second optical waveguides and the first and second dispersion slope compensating means, when optically coupled together, form a symmetric dispersion map. The transmission line portion components may further comprise a second optical amplifier connectable in optical communication with one or more of the aforementioned components, and the first and second optical waveguides. Desirably, one of the first and second optical amplifiers is provided at an end of the dispersion map and the other of the first and second optical amplifiers is provided towards the middle of the dispersion map.
The or each optical amplifier may be a fibre amplifier, such as an erbium doped fibre amplifier, or a fibre Raman amplifier. The or each optical amplifier may alternatively be a semiconductor optical amplifier device.
The first and second dispersion slope compensating means preferably comprise sections of a third optical waveguide, which is most preferably a second optical fibre. The second optical fibre may be a compensating fibre, such as dispersion compensating fibre or reverse dispersion fibre.
The first and second dispersion slope compensating means may alternatively or additionally comprise sections of a fourth optical waveguide including an optical grating. The fourth optical waveguide is preferably third optical fibre. The optical grating is preferably an optical fibre grating, such as a fibre Bragg grating.
According to a third aspect of the present invention there is provided an optical pulse transmission line having dispersion slope compensation, the optical pulse transmission line comprising a plurality of optical pulse transmission line portions, according to any of paragraphs 6 to 17 above, connected in optical communication in series.
The optical pulse transmission line is preferably for inclusion in a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero signal.
According to a fourth aspect of the present invention there is provided an optical pulse transmission line element having dispersion compensation and dispersion slope compensation, the transmission line element comprising:
a first optical waveguide of a first length (L1) having a first dispersion parameter (D1) and a first dispersion slope parameter (S1),
first dispersion slope compensating means of a second length (L2) having a second dispersion parameter (D2) and a second dispersion slope parameter (S2), and
second dispersion slope compensating means of a third length (L3) having a third dispersion parameter (D3) and a third dispersion slope parameter (S3),
the first optical waveguide, the first dispersion slope compensating means and the second dispersion slope compensating means being connected together in optical communication to form the transmission line element,
wherein the path average dispersion slope of the transmission line element is substantially zero and is given by
 less than S greater than =S1L1+S2L2+S3L3
and the path average dispersion ( less than D greater than ) of the transmission line element, having a total length L=L1+L2+L3 is given by       ⟨    D    ⟩    =                                                        D              2                        ⁢                          S              3                                -                                    D              3                        ⁢                          S              2                                                            S            3                    -                      S            2                              ⁢              (                  1          -                                    L              1                        L                          )              +                            L          1                L            ⁢                                                                  D                1                            ⁢                              (                                                      S                    3                                    -                                      S                    2                                                  )                                      +                                          (                                                      D                    2                                    -                                      D                    3                                                  )                            ⁢                              S                1                                                                        S              3                        -                          S              2                                      .            
The path average dispersion of the transmission line element is preferably wavelength independent.
Preferably, +/xe2x88x92[D1(S3xe2x88x92S2)+(D2xe2x88x92D3)S1]+/xe2x88x92(S3xe2x88x92S2) is greater whereby the path average dispersion may be controlled by selection of the length (L1) of the first optical waveguide. The path average dispersion of the transmission line element is desirably substantially zero. The path average dispersion of the transmission line element may alternatively be non-zero.
The optical pulse transmission line element is preferably for inclusion in a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero signal.
The first optical waveguide is preferably a first optical fibre. The first optical fibre may be a transmission fibre, such as standard monomode fibre, Lucent 2-wave fibre or Alcatel Terralight fibre.
The first dispersion slope compensating means is preferably a second optical waveguide. The second optical waveguide is desirably a second optical fibre, which may be a compensating fibre such as reverse dispersion fibre or dispersion compensating fibre. The first dispersion slope compensating means may comprise two parts, each part comprising a section of the second optical fibre.
The second dispersion slope compensating means preferably comprise a third optical waveguide. The third optical waveguide is desirably a third optical fibre, which may be a compensating fibre such as dispersion compensating fibre or reverse dispersion fibre.
The second dispersion slope compensating means may alternatively or additionally comprise a fourth optical waveguide including an optical grating. The fourth optical waveguide is preferably a fourth optical fibre. The optical grating is preferably an optical fibre grating, such as a fibre Bragg grating.
The transmission line element desirably further comprises at least one optical amplifier.
Desirably, the transmission line element comprises a plurality of sections of the first optical waveguide.
A first section of the first optical waveguide and the first dispersion slope compensating means are preferably arranged to form an optical pulse transmission line portion according to the first aspect of the invention.
The transmission line element desirably comprises:
first and second sections of the first optical waveguide, first and second parts of the first dispersion slope compensating means and first and second optical amplifiers, all being arranged to form a first optical pulse transmission line portion according to the first aspect of the invention, having a symmetric dispersion map;
optically coupled in series to
a second optical pulse transmission line portion according to the first aspect of the invention comprising the second dispersion slope compensating means, a third section of the first optical waveguide, and a third optical amplifier.
The or each optical amplifier is preferably a fibre amplifier, such as an erbium doped fibre amplifier or a fibre Raman amplifier. The or each optical amplifier may alternatively be a semiconductor optical amplifier device.
Desirably, a plurality of first optical pulse transmission line portions according to the first aspect of the invention, having a symmetric dispersion map, are provided, connected together in series in optical communication.
The path average dispersion slope of the or each first optical pulse transmission line portion is desirably substantially zero. The path average dispersion of the or each first optical pulse transmission line portion is desirably non zero and of a first sign.
The path average dispersion slope of the second optical pulse transmission line portion is desirably substantially zero. The path average dispersion of the second optical pulse transmission line portion is desirably non zero and of the opposite sign.
The path average dispersion of the optical pulse transmission line element is preferably equal to the sum of the path average dispersion of the or each first optical pulse transmission line portion and the path average dispersion of the second optical pulse transmission line portion. The path average dispersion of the transmission line element is desirably substantially zero.
According to a fifth aspect of the present invention there are provided optical pulse transmission line element components for incorporation into an existing terrestrial communication line which comprises a first optical waveguide of a first length (L1) having a first dispersion parameter (D1) and a dispersion slope parameter (S1) of a first sign, the components comprising:
first dispersion slope compensating means of a second length (L2) having a second dispersion parameter (D2) and a second dispersion slope parameter (S2), and
second dispersion slope compensating means of a third length (L3) having a third dispersion parameter (D3) and a third dispersion slope parameter (S3),
the first optical waveguide, the first dispersion slope compensating means and the second dispersion slope compensating means being connectable together in optical communication to thereby form a transmission line element,
wherein the path average dispersion slope of the transmission line element is substantially zero and is given by
 less than S greater than =S1L1+S2L2+S3L3
and the path average dispersion ( less than D greater than ) of the transmission line element, having a total length L=L1+L2+L3 is given by       ⟨    D    ⟩    =                                                        D              2                        ⁢                          S              3                                -                                    D              3                        ⁢                          S              2                                                            S            3                    -                      S            2                              ⁢              (                  1          -                                    L              1                        L                          )              +                            L          1                L            ⁢                                                                  D                1                            ⁢                              (                                                      S                    3                                    -                                      S                    2                                                  )                                      +                                          (                                                      D                    2                                    -                                      D                    3                                                  )                            ⁢                              S                1                                                                        S              3                        -                          S              2                                      .            
The path average dispersion of the transmission line element is preferably wavelength independent.
Preferably, +/xe2x88x92[D1(S3xe2x88x92S2)+(D2xe2x88x92D3)S1]+/xe2x88x92(S3xe2x88x92S2) is greater whereby the path average dispersion may be controlled by selection of the length (L1) of the first optical waveguide. The path average dispersion of the transmission line element is desirably substantially zero.
The optical pulse transmission line element preferably forms part of a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero signal.
The first optical waveguide is preferably a first optical fibre. The first optical fibre may be a transmission fibre, such as standard monomode fibre, Lucent 2-wave fibre or Alcatel Terralight fibre.
The first dispersion slope compensating means is preferably a second optical waveguide. The second optical waveguide is desirably a second optical fibre, which may be a compensating fibre such as reverse dispersion fibre or dispersion compensating fibre. The first dispersion slope compensating means may comprise two parts, each part comprising a section of the second optical fibre.
The second dispersion slope compensating means preferably comprises a third optical waveguide. The third optical waveguide may be a third optical fibre, which may be a compensating fibre, such as dispersion compensating fibre.
The second dispersion slope compensating means may alternatively or additionally comprise a fourth optical waveguide including an optical grating. The fourth optical waveguide is preferably a fourth optical fibre. The optical grating is preferably an optical fibre grating, such as a fibre Bragg grating.
The transmission line element components desirably further comprise at least one optical amplifier, connectable in optical communication with one or more of the above mentioned components and the first optical waveguide.
The existing terrestrial communication line may comprise a plurality of sections of the first optical waveguide.
The first dispersion slope compensating means is preferably connectable in optical communication to a first section of the first optical waveguide, to thereby form an optical pulse transmission line portion according to the first aspect of the invention.
The transmission line element components and the existing terrestrial communication line are desirably connectable together in optical communication to thereby form a transmission line element which comprises:
first and second sections of the first optical waveguide, first and second parts of the first dispersion slope compensating means, and first and second optical amplifiers, connected together in optical communication to thereby form a first optical pulse transmission line portion according to the first aspect of the invention, having a symmetric dispersion map;
optically coupled in series to
a second optical pulse transmission line portion according to the first aspect of the invention comprising the second dispersion slope compensating means, a third section of the first optical waveguide, and a third optical amplifier connected together in optical communication.
The or each optical amplifier is preferably a fibre amplifier, such as an erbium doped fibre amplifier, or a fibre Raman amplifier. The or each optical amplifier may alternatively comprise a semiconductor amplifier device.
Desirably, the transmission line element components and the existing terrestrial communication line are connectable together in optical communication to thereby form a plurality of first optical pulse transmission line portions according to the first aspect of the invention, each having a symmetric dispersion map.
The path average dispersion slope of the or each first optical pulse transmission line portion is desirably substantially zero. The path average dispersion of the or each first optical pulse transmission line portion is desirably non zero and of a first sign.
The path average dispersion slope of the second optical pulse transmission line portion is desirably substantially zero. The path average dispersion of the second optical pulse transmission line portion is desirably non zero and of the opposite sign.
The path average dispersion of the optical pulse transmission line element is preferably equal to the sum of the path average dispersion of the or each first optical pulse transmission line portion and the path average dispersion of the second optical pulse transmission line portion. The path average dispersion of the transmission line element is desirably substantially zero.
According to a sixth aspect of the invention there is provided an optical pulse transmission line having dispersion compensation and dispersion slope compensation, the transmission line comprising a plurality of transmission line elements according to any of paragraphs 32 to 49 above connected together in optical communication in series.
The optical pulse transmission line is preferably for inclusion in a wavelength division multiplexed optical pulse transmission system.
The optical pulse is desirably a return-to-zero optical pulse. The optical pulse is desirably an optical soliton. The optical pulse may alternatively be a chirped return-to-zero signal.
According to a further aspect of the present invention there is provided a method of manufacturing a pair of optical waveguides for inclusion in an optical pulse transmission line portion according to the first aspect of the present invention, the method including the steps of:
selecting the desired average dispersion ( less than D greater than ) of the transmission line portion; and
then selecting
the dispersion parameter (D1) of the first waveguide of the pair,
the dispersion slope parameter (S1) of the first waveguide,
the dispersion parameter (D2) of the second waveguide of the pair, and
the dispersion slope parameter (S2) of the second waveguide,
such that the following relationship is satisfied       ⟨    D    ⟩    =                              D          1                ⁢                  S          2                    -                        D          2                ⁢                  S          1                                    S        2            -              S        1            
Preferably, each optical waveguide comprises an optical fibre. Each optical waveguide may alternatively comprise a semiconductor waveguide.
The average dispersion of the transmission line portion may be anomalous, zero or normal. The average dispersion is preferably small, typically less than 1 picosecond/nanometer/kilometer.